Since III-V semiconductors have a high breakdown field and a high saturated electron mobility, it is expected that semiconductor devices comprising III-V semiconductors will have a high breakdown voltage and will control large currents. Current research includes research on semiconductor devices that have a heterostructure comprising gallium nitride (GaN), an example of which is disclosed in Japanese Laid-Open Patent Application Publication No. 2003-59946.
One of this type of semiconductor devices is an HEMT (High Electron Mobility Transistor) having a heterostructure comprising a p-GaN layer, and an n-AlGaN layer stacked on a top surface of the p-GaN layer. Since aluminum (Al) is contained in semiconducting crystals of the n-AlGaN layer, a band gap of this layer is wider than the p-GaN layer. A drain electrode, a gate electrode, and a source electrode are formed at a top surface of the n-AlGaN layer. The gate electrode is formed between the drain electrode and the source electrode.
In this type of HEMT, a potential well is formed by the p-GaN layer and the n-AlGaN layer at their junction, however, an energy level of conduction band of the potential well is above the Fermi level unless a positive gate voltage is not applied to the gate electrode. Consequently 2DEG (2 Dimensional Electron Gas) is not generated in the potential well while the gate voltage is not being applied to the gate electrode. As a result, normally-off operation of the HEMT is possible. While a predetermined on-voltage is applied to the gate electrode, the energy level of conduction band of the potential well becomes lower than the Fermi level, and the 2DEG is generated in the potential well. Since electrons in the 2DEG move within the potential well, electric currents flow between the drain electrode to the source electrode while the predetermined gate voltage is being applied to the gate electrode of HEMT.